Multiplexing and offset correction system for an image sensor array

ABSTRACT

An imaging apparatus includes two subsets of photosensors, the two subsets being interleaved along a linear array. Each photosensor is connectable, by the operation of a shift register, to a reference line and a signal line, to permit double-sampling of signals therefrom. Each subset of photosensors is associated with its own reference line and signal line, and signals from the two subsets of photosensors can be read out largely simultaneously.

INCORPORATION BY REFERENCE

The present application incorporates by reference U.S. Pat. Nos.5,638,121; 6,445,413; and 6,853,402, all assigned to the assigneehereof, each in its entirety.

TECHNICAL FIELD

The present invention relates to image sensor arrays such as used inraster input scanners or digital copiers. In particular, the inventionrelates to photosensitive chips wherein photosensors output signals ontoa video line.

BACKGROUND

Image sensor arrays typically comprise a linear array of photosensorswhich raster scan an image-bearing document and convert the microscopicimage areas viewed by each photosensor to image signal charges.Following an integration period, the image signal charges are amplifiedand transferred as an analog video signal to a common output line or busthrough successively actuated multiplexing transistors.

Although most scanning systems currently in use are ultimately digitalsystems, the “raw signal” coming out of the photosensors during thescanning process is an analog video signal, with the voltage magnitudecorresponding to the intensity of light impinging on the photosensor ata given time. Thus, when signals are read out from the photosensors on achip to be converted to digital data, different video levels,corresponding to the brightness of the reflected area being scanned by aparticular photosensor at a particular moment, are output as a series ofanalog voltage levels.

In order to increase the readout speed of image signals from, forexample, a linear array of photosensors, it is known to provide separate“odd” and “even” channels for the output of image signals. A basicexample of this technique is shown in U.S. Pat. No. 5,638,121. In brief,alternate photosensors along a linear array respectively output imagesignals into separate odd and even video lines, and these video linesare subsequently multiplexed, thus yielding a single video stream.

FIG. 1 is a schematic view showing the basic elements of a readoutsystem according to a prior-art implementation, illustrating an“odd-even” readout principle. There is provided on, for instance, aphotosensor chip, a set of photosensors 10 a-10 z, which are connectedby respective transistor switches 14 a, 14 b, etc. The switches in turnare activated by a shift register 18, which includes a set of stages(or, more precisely in the embodiment, half-stages) 20 a, 20 b, etc.,arranged along a single line 22, and activated by a pixel clock line 24.When a digital 1 is passed through the stages 20 a, 20 b, etc., signalsheld on the photosensors 10 a . . . 10 z are caused to be read out ofthe system in linear order, to form a line of useable image data, suchas for a digital scanner or copier.

According to this FIG. 1 example, the photosensors 10 a . . . 10 z arearranged in an interleaved manner with odd and even subsets, with theodd subsets of photosensors such as 10 a and 10 c connected to an oddvideo line 12 a, and the even photosensors such as 10 b and 10 d,connected to an even video line 12 b. Video line 12 a receives the videooutputs only of the odd photosensors, and the even video line 12 breceives the video outputs only of the even photosensors. Because boththe odd and even photosensors are controlled by a single shift register18, having half-stages 20 a, 20 b, etc., the video voltage signals froma set of odd and even pixels can together be read out onto the odd andeven video lines at a considerably faster rate than in a situation whereall of the photosensors are reading out to a single video line. Anotherpractical advantage is that, because fewer transistors are connected toeach of the odd and even lines, there is less capacitance on each linethan if both odd and even signal trains were read out on one line, andeach video signal can settle to its final value faster.

With any sophisticated system for reading out images signals from aseries of photosensors, a common practical problem is known as “darknon-uniformity” (DNU) or “fixed-pattern noise.” With each individualphotosensor for an associated transfer circuit, there is likely to be asingle dedicated amplifier (a “pixel amplifier”). Given thepracticalities of constructing photosensors and circuits on a chip, itis likely that certain amplifiers, associated with certain photosensors,will consistently have higher output relative to other amplifiersassociated with other photosensors. DNU is defined as the maximumdifference in output voltage between any two pixels of an image sensorwhile in the dark. There exist basic techniques for overcoming DNU, suchas mentioned in U.S. Pat. No. 5,654,755.

“Double sampling” is a technique that can be used to reduce the DNUcontribution of the pixel amplifier. With this concept, the output ofeach pixel amplifier is sampled twice, once with the optical signal fromthe photosensor such as 10 a and once with a common reference signal, sothat the output signal from the pixel is defined as the differencebetween the two samples. Additional signal processing stitches the videoback together and restores the output level. If the pixel amplifieroffset is constant, the subtraction of the double samples, to the firstorder, eliminates its contribution to DNU. However, the problem withdoing double sampling in a standard architecture is that the pixelamplifier is read out twice to the same video line, which effectivelyreduces the output data rate by 50%.

The present disclosure relates to a photosensor circuit architecturethat enables double sampling of video outputs without a necessarydecrease in output data rate.

SUMMARY

According to one aspect, there is provided an imaging apparatus,comprising a first subset of photosensors, and a second subset ofphotosensors. A plurality of pixel amplifiers is provided, at least onepixel amplifier being associated with each photosensor in the firstsubset of photosensors and the second subset of photosensors. A shiftregister includes a plurality of stages, at least one of the pluralityof stages associated with each photosensor in the first subset ofphotosensors and the second subset of photosensors. A first referenceline is associated with the first subset of photosensors, and isconfigured to read a reference signal associated with each pixelamplifier associated with the first subset of photosensors. A firstsignal line is associated with the first subset of photosensors, and isconfigured to read a signal associated with each pixel amplifierassociated with the first subset of photosensors. A second referenceline is associated with the second subset of photosensors, and isconfigured to read a reference signal associated with each pixelamplifier associated with the second subset of photosensors. A secondsignal line is associated with the second subset of photosensors, and isconfigured to read a signal associated with each pixel amplifierassociated with the second subset of photosensors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the basic elements of a readoutsystem according to a prior-art implementation, illustrating an“odd-even” readout principle.

FIG. 2 is a detailed schematic view showing circuitry associated withtwo adjacent photosensors in an embodiment.

FIG. 3 is a functional timing diagram, with a typical output response,for the various lines as labeled in FIG. 2, as the apparatus of FIG. 2is operated. The various signal forms in FIG. 3 are labeled tocorrespond to similarly-labeled nodes in the FIG. 2 circuit, even thoughnot all such labeled nodes are described in the below text.

As used in the below description, and notwithstanding other oradditional labeling of elements in the Figures, the convention “pixel N”will mean a given photosensor and associated circuitry, while, forinstance, “pixel N+1” will mean an adjacent photosensor and circuitryalong a readout direction of the linear array, “pixel N−2” will mean twophotosensors upstream along a readout direction of the linear array,etc.

DETAILED DESCRIPTION

To implement “double sampling” without affecting output data rate, thedisclosure proposes splitting the odd and even video lines into signaland reference pairs.

FIG. 2 is a detailed schematic view showing circuitry associated withtwo adjacent photosensors. such as 10 a and 10 b in FIG. 1, in anembodiment: between FIG. 1 and FIG. 2, equivalent elements have the samereference number. In a practical implementation of the FIG. 2 structure,the photosensors 10 a and 10 b would respectively represent a single“odd” and a single “even” photosensor, as those two types ofphotosensors would be interleaved along a long linear array havinghundreds or thousands of photosensors.

In FIG. 2, each photosensor 10 a and 10 b sends an exposure-based charge(perhaps through a transfer circuit, not shown) to a respective resetnode 11 a, 11 b. In addition to the photosensor 10 a or 10 b andassociated shift register stage (or, more specifically in thisembodiment, half-stages) 20 a, 20 b, there is provided in the FIG. 2embodiment a unity-gain pixel amplifier 30 a, 30 b, as well as anadditional shift register stage 21 a, 21 b, for each photosensor.

Associated with the photosensors are what can be generally called“readout lines” indicated as VID_(O)[S] and VID_(O)[R], associated withall odd photosensors such as 10 a; and VID_(E)[S] and VID_(E)[R],associated with all even photosensors such as 10 b. These readout lineshave the same overall function of reading out image-based signals, asdescribed above in regard to lines 12 a and 12 b in FIG. 1. However,these two lines per photosensor are configured to carry out a “doublesampling” of each pixel readout from each photosensor. As mentionedabove, with double sampling, the output of each pixel amplifier issampled twice, once with the optical signal from the photosensor such as10 a and once with a common reference signal, so that the output signalfrom the pixel is defined as the difference between the two samples.

Taking photosensor 10 a as an example, the lines VID_(O)[S] andVID_(O)[R] are configured relative to the unity-gain pixel amplifier 30a to facilitate double sampling. The “reference” line VID_(O)[R], tappedto the output of pixel amplifier 30 a, effectively receives and outputsa reference signal relating to the “dark” output from pixel amplifier 30a, i.e., the signal output from pixel amplifier 30 a when there is nosignal from the associated photosensor 10 a. The “signal” lineVID_(O)[S], tapped between the pixel select line from shift registerstage 20 a and the negative input to pixel amplifier 30 a, reads anoptical-signal output from the pixel amplifier 30 a (based ultimately onthe image-based charge for photosensor 10 a at a given time). As can beseen, the lines VID_(O)[S] and VID_(O)[R] associated with pixelamplifier 30 a are respectively connected to certain stages in the shiftregister, for activation when a shift register signal passes through thesystem along line 22.

To effect the double sampling operation, the line VID_(O)[R], whenactivated via shift register stage 20 b, outputs a reference signalrelating to the “dark” output from pixel amplifier 30 a. The lineVID_(O)[S], when activated via shift register stage 20 a, reads anoptical-signal output from the pixel amplifier 30 a. When the referencesignal from line VID_(O)[R] is subtracted from the optical-signal outputon line VID_(O)[S], the remainder represents a signal in which the darknon-uniformity (DNU) associated with that particular pixel amplifier 30a is subtracted out.

In the illustrated embodiment, the readout sequence over time for eachpair of photosensors such as shown in FIG. 2, as a shift register signalpasses through the shift register stages 20 a, 21 a, 20 b, 21 b, is:S(odd pixel), S(even pixel), R(odd pixel), R(even pixel). To carry outthis readout, any pair of odd and even photosensors such as 10 a and 10b, with their respective associated amplifiers 30 a and 30 b, interactwith connections to two photosensors in either direction (pixels (N−2 toN+2)). Thus, in further detail, the PX_(SEL) output of the shiftregister stage such as 22 a of pixel (N) will connect the signal outputof the pixel amplifier 30 a to the signal video line and the referencesignal of pixel (N−2) to the reference video line. The PX_(SEL) of pixel(N+2) will then connect the reference output of the PA from pixel (N) tothe reference video line. The video line pairs and second video lineswitch allow two odd pixels and two even pixels to be sampledsimultaneously. As a result, “double sampling” with the embodiment willnot double the readout time or correspondingly reduce the output datarate.

Cumulatively, the DNU for each individual pixel amplifier in the arrayis thus removed with each signal readout. The subtraction operationbetween VID_(O)[S] and VID_(O)[R] for each pixel in each readoutoperation is carried out by a downstream system including specializedcircuitry and/or software, generally shown as 40, which also performsthe necessary multiplexing of the odd and even signals. As in theembodiment described in U.S. Pat. No. 5,638,121, the downstream videopath is required to do additional processing to stitch and restore thevideo; but there are no additional logic gates required because thePIX_(SEL) signals from the shift register stages can be used directlyand do not need to be conditioned by a pixel clock. As a result, thereis minimal impact to the width of the sensor chip even with the secondvideo line switch for each pixel and the two additional video lines forthe pixel array.

FIG. 3 is a functional timing diagram, with a typical output response,for the various lines as labeled in FIG. 2, as the apparatus of FIG. 2is operated via a succession of signals passing through the series ofshift register stages 20 a, 21 a, 20 b, 21 b, etc.

In combination with analogous hardware and control associated with theeven photosensors such as photosensor 10 b, the signal line VID_(E)[S]and reference line VID_(E)[R] configured relative to the unity-gainpixel amplifier such as 30 b, etc., there is thus provided a system bywhich separate signal and reference lines, through which double samplingis possible, are provided for separate subsets of photosensors andassociated pixel amplifiers. Because the “odd” and “even” subsets can beread out simultaneously, the double sampling, which largely obviates DNUfrom the whole apparatus, is enabled without impacting the overallreadout rate (as is common in double-sampling arrangements) and also hasimpact on “real estate” on a photosensor chip is minimal.

Although the above-described embodiment is shown in the context of alinear array of photosensors as would be used in a digital copier orscanner, the teachings herein can readily be adapted for use in atwo-dimensional photosensor array. Although the color aspects of thedescribed embodiment are not discussed, the teachings herein can readilybe adapted for a full-color device. Although the described embodimentshows “odd” and “even” subsets of photosensors along an array,interleaved on a one-by-one basis, the terms “odd” and “even” shall beconstrued broadly to encompass any arrangement of subsets ofphotosensors in a device, no matter to what extent the photosensors inthe subsets are interleaved 9 (e.g., the two subsets could be entirelyseparate from each other on the chip). The teachings can also be adaptedfor embodiments in which there are more than two subsets of photosensorsin an apparatus (e.g., the outputs of four subsets of photosensors couldbe multiplexed, to increase the readout rate).

The claims, as originally presented and as they may be amended,encompass variations, alternatives, modifications, improvements,equivalents, and substantial equivalents of the embodiments andteachings disclosed herein, including those that are presentlyunforeseen or unappreciated, and that, for example, may arise fromapplicants/patentees and others.

1. An imaging apparatus, comprising: a first subset of photosensors, anda second subset of photosensors; a plurality of pixel amplifiers, atleast one pixel amplifier being associated with each photosensor in thefirst subset of photosensors and the second subset of photosensors; ashift register comprising a plurality of stages, at least one of theplurality of stages associated with each photosensor in the first subsetof photosensors and the second subset of photosensors; a first referenceline, associated with the first subset of photosensors, configured toread a reference signal associated with each pixel amplifier associatedwith the first subset of photosensors; a first signal line, associatedwith the first subset of photosensors, configured to read a signalassociated with each pixel amplifier associated with the first subset ofphotosensors; a second reference line, associated with the second subsetof photosensors, configured to read a reference signal associated witheach pixel amplifier associated with the second subset of photosensors;and a second signal line, associated with the second subset ofphotosensors, configured to read a signal associated with each pixelamplifier associated with the second subset of photosensors.
 2. Theapparatus of claim 1, further comprising a downstream system forobtaining a difference between signals on the first reference line andfirst signal line, and between signals on the second reference line andsecond signal line.
 3. The apparatus of claim 1, further comprising adownstream system for multiplexing signals associated with the firstreference line, first signal line, second reference line, and secondsignal line.
 4. The apparatus of claim 1, the first reference line,first signal line, second reference line, and second signal line beingoperated by stages of the shift register.
 5. The apparatus of claim 4,wherein the stages of the shift register are arranged with the firstreference line, first signal line, second reference line, and secondsignal line so that the lines read out, in time sequence, a signal froma photosensor in the first subset, a signal from a photosensor in thesecond subset, a reference signal associated with a photosensor in thefirst subset, and a reference signal associated with a photosensor inthe second subset.
 6. The apparatus of claim 1, the first reference linebeing tapped to the outputs of each of the pixel amplifiers associatedwith the first subset of photosensors, and the second reference linebeing tapped to the outputs of each of the pixel amplifiers associatedwith the second subset of photosensors.
 7. The apparatus of claim 1, thefirst signal line being tapped between a stage of the shift register andan input to each of the pixel amplifiers associated with the firstsubset of photosensors, and the second signal line being tapped betweena stage of the shift register and an input to each of the pixelamplifiers associated with the second subset of photosensors.
 8. Theapparatus of claim 1, the first subset of photosensors being interleavedwith the second subset of photosensors along a linear array.